In oil prospecting and extraction it is common practice to use metallic pipes of a commercial length, which are made-up to form a string long enough to reach the depths in which oil or gas reservoirs or pools are located. The depth of reservoirs is increasing constantly and therefore the joints have to be designed in such a manner that they can withstand higher loads and stresses while ensuring good sealing properties.
An improvement of the performances of threaded joints is achieved by appropriate design of all load bearing surfaces of the pin and the box, i.e. thread flanks, abutment shoulders etc., to make them maintain reciprocal contact with appropriate structural and dimensional interference in all load ranges under which the joints are expected to operate. This depends on the intensity of torque that can be applied, whereby torque intensity is determined by the difference between the value of yield torque and the value of shoulder torque.
However, the provision of too high an interference is detrimental for make-up operation, since this can cause seizure or galling of the joint. Efforts have been made to reach a good balance between having a low make-up torque and having high yield torque, which is a condition for withstanding high loads and having good sealing properties.
On one side, to avoid seizure during make-up, state of the art technology generally uses a fluid lubricant, e.g. grease or dope.
Efforts are also made to increase yield torque limit, which has always been an important issue for research in this technical field. Yield torque (Y.T.) was increased generally by modifying the joint design, but it has now come close to its technical limit for doped joints.
Use of dope in turn has drawbacks and would preferably be avoided. The risks posed by the use of dope include: dirt entrapment; non-uniform distribution of dope because of manual operation, which might also cause overdoping in all or part of the joint; and cleaning of the joints that has to be carried out in the oilfield in adverse environmental conditions.
Therefore, to reduce costs and improve operations dope free solutions for joints have been developed and have been in use for several years. Dope-free solutions have achieved a good torsional strength, but in some cases they have drawbacks because the make-up torque can reach high values. In such cases make-up torque of dope free solutions for joints is usually lowered by adding oils or contaminants, thus losing partly or totally the advantages of dope free techniques and causing the risk of reducing also the value of yield torque and of overtorquing the joint.
Dope free solutions for joints have been disclosed in several patents, e.g. U.S. Pat. No. 4,414,247, U.S. Pat. No. 6,027,145, and U.S. Pat. No. 4,692,988. They use a single anti-seize coating instead of dope with the same friction factor in all points of the joint surface. An appropriate friction value must also be chosen for such joints, because an increase of the friction factor to enhance the yield torque value increases also the shoulder torque, whereas a reduction of the friction factor allows performing make-up with values of shoulder torque similar to that of known doped joints but reduces its yield torque value. The consequences in both cases are that the value of torque is low and unsatisfactory.